The system will always create separate invoices for each separate animal, but VetBadger has a feature that will let you "merge" those invoices once you're done with all of the exams. You can find this feature on the client's page, if you scroll down to the "Outstanding Invoices" section. Each invoice will have a checkbox next to it, like this:
If you select a few invoices and click the "Merge Invoices" button you'll see this window:
Once you've clicked the "Merge" button the system will instruct QuickBooks to create a single new merged invoice and void all the source invoices.